VULCANISATION 101
contains bromine in less proportion than is required to form the tetra-
bromide, the balance being made up of the equivalent of sulphur. Similarly
in the formation of the nitrosite coefficients with increasing vulcanisation,
decreasing proportions of nitrous gases are taken up. Harries * has stated
that sulphur does not pass into the hydrochloride prepared from vulcanised
rubber, but this is in reference to a sample which was said to yield all its
sulphur to acetone on extraction. On keeping in a warm place for a long time
the sulphur becomes unextractable and is found in the hydrochloride prepared
in the usual way. From this we may conclude that vulcanised rubber, in
the ordinarily accepted sense of the term, yields a hydrochloride in which
sulphur is present in combination.
It is also interesting to note that the hydrocaoutchouc prepared by Staud-
inger and Fritschi (see p. 85) by hydrogenating caoutchouc in presence of a
platinum catalyst is not capable of vulcanisation, and it appears logical to
conclude that this is due to the elimination of the double bonds, as a result of
which a sulphur addition product cannot be formed. The hydrogenation was,
however, carried out at a temperature of 270° C., under which conditions it is
unlikely that the caoutchouc molecule would remain unaffected. It would
be of greater interest to know whether the hydrocaoutchouc prepared by
hydrogenation at the lower temperature employed by Pummerer and Burkard
(loc. cit.) is capable of vulcanisation, but this has not yet been investigated.
While the available evidence is overwhelmingly in favour of the occurrence
of a chemical reaction during vulcanisation, it can hardly be said that the
possibility of a physico-chemical theory of vulcanisation is entirely excluded.
What does appear to be proved is that the process is not one consisting merely
of an adsorption of sulphur by rubber. It may be noted that a study of the
velocity of extraction of sulphur from vulcanised rubber by means of acetone
led Spence f to conclude that part of the " free " sulphur was adsorbed and
that possibly adsorption preceded the chemical reaction. The difficulty of
removing the last portions of sulphur may, however, conceivably be due to the
slowness of penetration of the interior portions of the rubber particles by the
acetone, which would lead to a decrease in velocity of extraction from the
surface inwards. If it is accepted that heating with a sufficiently high propor-
tion of sulphur leads to the formation of a definite compound, there is no reason
to suppose that this reaction does not take place when proportions of sulphur
such as are used in producing soft rubber goods, i.e. less than 10 per cent., are em-
ployed. The fixation of sulphur up to 32 per cent. takes place uniformly, and there
is no indication that at any particular stage the reaction changes in character.
* Ber., 1916, 49, 1196. t &<>& Zeto., 1911, 9, 300.